@Herbert: Word/syllable count and sentence length work well as a measure of readability for both native and non-native speakers though. Whether or not they were born to it, a user’s capacity to process what they are reading has a huge effect on the effectiveness of our sites. Best to tackle both user groups with an simple tool like Flesch-Kincaid, right?
Colloquialisms, slang, and other culturally dense language isn’t something I touched on here, but you make a good point. I think as a matter of course, we need to minimize our use of that language in our web/marketing copy. Even regional differences in English speaking populations can mean some head-scratching.